A method of manufacturing a tubular discharge vessel that has been bent several times and is intended for an electric discharge lamp is known from U.S. Pat. No. 4,319,162, HOEH, and the corresponding German Patent Disclosure Document No. 30 05 052. The bends are made by heating a substantially straight glass tube at the region intended for bending until the glass tube softens, whereupon the heated region of the glass tube is bent about a mold by moving the non-heated tube sections in the respective directions.
U.S. Pat. No. 4,481,442, ALBRECHT et al., and corresponding European Patent Specification No. 61 758, also describe a method of manufacturing a tubular discharge vessel bent once or several times and intended for a low-pressure mercury vapor discharge lamp. The tube sections of a straight glass tube that are to be bent are heated to softening temperature, and the 180.degree.-bend is manufactured by bending the adjacent tube sections towards one another. Thereafter, the 180.degree.-bend is placed into a mold, compressed air is blown into the tube ends, and there is provided a U-shaped bend corresponding to the mold and having substantially rectangular corners. The rectangular corners serve as the cold spots for the optimal operation of the lamp.
In the manufacture of the 180.degree.-bends, the wall thickness of the glass tube decreases at the outer surface area of the bend owing to the bending operation itself. When a 180.degree.-bend is made that has substantially rectangular corners, there is an even stronger decrease in wall thickness. Because of the thin tube wall, the discharge vessels are particularly subject to breakage at these locations.
This hazard of breakage may be eliminated when, in the manufacture of the discharge vessel, a glass tube is used whose wall is sufficiently thick in order that, upon manufacture of the 180.degree.-bends, the thickness of the glass wall at the outer surface area will not fall below a certain minimum thickness. However, a considerable increase in the weight of both the discharge vessel and the finished lamp, as well as higher costs, have to be accepted.